Logging while drilling (LWD) techniques are well-known in the downhole drilling industry and are commonly used to measure borehole and formation properties during drilling. Such LWD techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic caliper, downhole pressure, and the like. Many such LWD techniques require that the standoff distance between the logging sensors in the borehole wall be known with a reasonable degree of accuracy. For example, LWD nuclear/neutron techniques utilize the standoff distance in the count rate weighting to correct formation density and porosity data.
Measurement of the standoff distance is also well-known in the art. Conventionally, standoff measurements typically include transmitting an ultrasonic pulse into the drilling fluid and receiving the portion of the ultrasonic energy that is reflected back to the receiver from the drilling fluid borehole wall interface. The standoff distance is then typically determined from the acoustic velocity of the drilling fluid and the time delay between transmission and reception of the ultrasonic energy.
One drawback with such conventional standoff measurements is that the acoustic velocity of the drilling fluid can vary widely depending on the borehole conditions. For example, the presence of cuttings, hydrocarbons (either liquid or gas phase), and/or water in the drilling fluid is known to have a significant effect on both the acoustic velocity and the attenuation coefficient of the drilling fluid. Moreover both temperature and pressure are also known to have an effect on the acoustic velocity and attenuation coefficient of the drilling fluid. Typically only temperature and pressure changes are accounted in estimates of the acoustic velocity. In the current state-of-the-art, the acoustic velocity of the drilling fluid is estimated based on type of mud, salinity, downhole temperature and pressure measurements, and empirically derived equations (or lookup tables) that are based on laboratory measurements of the base drilling fluid. The presence or absence of cuttings, oil, water, and/or gas bubbles in the drilling fluid typically go unaccounted. Depending on the type of drilling fluid and on the concentration of cuttings, oil, water, and/or gas bubbles therein, the degree of error to the estimated acoustic velocity and attenuation coefficient can be significant. Moreover, as indicated above, such errors are not isolated, but can result in standoff distance errors, which can lead to subsequent LWD nuclear data weighting errors. Acoustic velocity errors can also have a direct affect on sonic LWD data quality. For example, in acoustically slow formations (where the formation shear velocity is less than the drilling fluid velocity), the borehole guided or flexural wave is present in the waveform. To determine the true formation shear velocity, the computed guided/flexural velocity typically needs to be corrected using a dispersion correction model. Errors in the acoustic velocity of the drilling fluid used in the model can therefore result in errors in formation shear velocity estimates.
Therefore, there exists a need for an apparatus and method for making real-time, in-situ (i.e., downhole) measurements of the acoustic velocity of the drilling fluid. Such measurements would potentially improve the reliability of downhole standoff/caliper measurements and nuclear and sonic LWD data. An apparatus and method for making real-time, in-situ measurements of the attenuation co-efficient of the drilling fluid would also be advantageous.